The present invention relates to a method for setting an expansion anchor by means of a power tool, in particular an impact screwdriver. The present invention further relates to a power tool, in particular an impact screwdriver performing this method, containing an input device for sensing a type of an expansion anchor or a tightening torque for the expansion anchor, a percussion mechanism for generating rotary impacts that can be transferred from and to the expansion anchor, a device for sensing a rotational angle of a drive shaft of the power tool and a control device.
Expansion anchors are used in a pre-drilled hole and are subsequently pre-stressed in the hole in a standard manner using a torque key. The use of a torque key has proven necessary since a user cannot see whether the expansion anchor expands correctly, i.e., is set when the expansion anchor is inserted into the hole. In this regard, problems occur in the case of excessively low expansion and thus lower resilience of the anchoring in the subgrade and also in the case of excessively high expansion, due to possible fatigue of the expansion anchor. The manufacturers of the expansion anchors accordingly indicate an associated tightening torque which should be adjusted on the torque key for setting.
A setting method for an expansion anchor and an impact screwdriver for setting an expansion anchor according to the prior art is for example disclosed in the German patent application DE 10 2011 005 079 A1. This document of the prior art describes a method for setting an expansion anchor by means of an impact screwdriver, in the case of which rotary impacts are repeatedly exerted on the expansion anchor. A repetition rate of the rotary impacts is adjusted in dependence on a tightening torque predefined for expanding an expansion sleeve of the expansion anchor. The impact screwdriver adjusts the exertion of rotary impacts when a sensed medium rotational speed at a nut of the expansion anchor falls below a threshold value.
In a series of tests, it was determined that the relaxation behavior of an expansion anchor is dependent on the setting speed by means of which the expansion anchor is positioned in a material (e.g., mineral subgrade). Relaxation here describes the gradual loss of pre-stress force of the expansion anchor set in a material due to setting effects.
In the case of expansion anchors that are set with high tightening speed, a very high decrease of the pro-stress force of the expansion anchor set in a material was observed in the first minutes after concluding the setting process. In the case of a conventional setting method (i.e., manually setting an expansion anchor in a material) with a torque key, this loss of pre-stress force is less pronounced. An increased loss of pre-stress force reduces the load values that can be achieved with the expansion anchor and thus limits the application field of the expansion anchor.
The loss of the pre-stress force is supposedly caused by the very high speed of the setting process. In the case of tightening by hand (i.e., in the case of manually setting an expansion anchor in a material), a torque is usually applied in a plurality of intervals over a period of time of approximately 20 seconds. In the breaks between these intervals, in which the return movement is performed with the torque key, local tension peaks in the material (e.g., concrete) may already be reduced. In the case of a further return movement or in the case of a further stroke by the torque key, the loss of pre-stress force resulting therefrom is again compensated.
In the case of setting using an impact screwdriver, the entire setting process of the expansion anchor is concluded within approximately 2 seconds. As a result, there is no time in order to reduce the tension peaks between the setting intervals. For this reason, the setting process takes place only after concluding the actual setting processes.
It is thus the object of the present invention to solve the problem described above and in particular to provide a method for setting an expansion anchor by means of a power tool, in particular an impact screwdriver, as well as to provide a power tool, in particular an impact screwdriver, for performing this method. The setting process of an expansion anchor is optimized by the method and the power tool for performing the method such that the expansion anchor can be subjected to the highest possible loading of tensile stress.
To this end, a method is provided for setting an expansion anchor by means of a power tool, in particular an impact screwdriver.
The method is characterized according to the invention by the steps:
exerting rotary impacts on an expansion anchor 1 by way of the power tool 20 to expand an expansion sleeve 3 of the expansion anchor 1 in dependence on a first rotational speed generated in the power tool 20 until a tightening torque generated in the power tool 20 and exerted on the expansion anchor 1 corresponds to a predetermined threshold value;
exerting a predetermined number of rotary impacts on the expansion anchor 1 by way of the power tool 20 to expand an expansion sleeve 3 of the expansion anchor 1 in dependence on a first rotational speed generated in the power tool 20; and
exerting rotary impacts on the expansion anchor 1 by way of the power tool 20 to expand the expansion sleeve 3 of the expansion anchor 1 in dependence on a second rotational speed, generated in the power tool 20 and reduced in comparison to the first rotational speed, for a predetermined period of time (t).
By exerting further rotary impacts on the already set expansion anchor with a reduced rotational speed, a re-stressing may be generated on the expansion anchor, with the result that the setting effects can be balanced out without the pre-stress force in the expansion anchor being increased further.
Furthermore, a power tool, in particular an impact screwdriver, is provided for performing this method, containing an input device for sensing a type of an expansion anchor or a tightening torque for the expansion anchor, a percussion mechanism for generating rotary impacts that can be transferred from and to the expansion anchor, a device for sensing a tightening torque of the power tool and a control device.
According to the invention, the power tool is characterized in that the control device is configured to set a first rotational speed generated in the power tool, with the result that rotary impacts that are dependent on the first rotational speed can be exerted on the expansion anchor to expand an expansion sleeve of the expansion anchor until a tightening torque generated in the power tool and exerted on the expansion anchor corresponds to a predetermined threshold value; to set the power tool such that a predetermined number of rotary impacts is exerted by the power tool on the expansion anchor to expand the expansion sleeve of the expansion anchor in dependence on the first rotational speed generated in the power tool, and to set a second rotational speed, generated in the power tool and reduced in comparison to the first rotational speed, with the result that for a predetermined period of time rotary impacts that are dependent on the second rotational speed can be exerted on the expansion anchor to expand the expansion sleeve of the expansion anchor.
Re-stressing may be hereby generated on the expansion anchor, with the result that the setting effects can be balanced out without the pre-stress force in the expansion anchor being further increased.
Further advantages emerge from the following description of the figures. Different exemplary embodiments of the present invention are illustrated in the figures. The figures, the description and the claims contain numerous features in combination. The person skilled in the art will also expediently consider the features individually and combine them to form reasonable further combinations.